Liquid-gas absorption process

ABSTRACT

Stable alkanolamine-acid gas reaction products formed during the absorption of acidic gases from gas mixtures by means of an alkanolamine-containing absorbent liquid are reverted to free alkanolamines by treatment with caustic and water. The alkanolamines thus formed are recovered from a two phase system and reused in the absorption process.

United States Patent Van Scoy 1 51 Apr. 25, 1972 [54] LIQUID-GASABSORPTION PROCESS [72] Inventor: Robert W. Van Seoy, Walnut Creek,Calif.

[73] Assignee: Shell Oil Company, New York, NY.

[22] Filed: Mar. 5, 1970 [21] Appl. No.: 16,800

[52] U.S. Cl. ..23/2 A [51] int. Cl ..B0ld 53/34 [58] Field of Search..23/2, 2 A, 3, 3 LA; 260/584 R [56] References Cited UNITED STATESPATENTS 2,785,045 3/1957 Shen Wu Wah et al. ..23/2

RECLAIMING COLUMN CONTAMINATEDS SOLVE NT 6 WATER I AQUEOUS CAUSTICREBOILER q 2,701,750 2/1955 Paulsenetal ..23/2 3,535,260 10/1970 Singh..23/2AX Primary Examiner -Earl C Thomas Attorney-Harold L. Denkler andGlen R. Grunewald s7 Ans'mAcT Stable alkanolamine-acid gas reactionproducts formed during the absorption of acidic gases from gas mixturesby means of an alkanolamine-containing absorbent liquid are reverted tofree alkanolamines by treatment with caustic and water. Thealkanolamines thus, formed are recovered from a,two phase system andreused in the absorption process.

6 Claims, 2 Drawing Figures 5 ALKANOLA- I 14 MINE PHASE SEPARATIONCOLUMN 3 REVERT- ED ALKANQ' LAMINE PHASE STRIPPER REBOILER PatentedApril 2-5, 1972 2 Shoots-Shoo. z

ou z N wE oo z 030m w mmsi wzo o r 8 om 259% 206555 M211 mzo 5.96 ZEEwQ$2 @503 N oo z Q30 $25 052 INVENTOR:

ROBERT W. VAN SCOY BY:

HIS ATTORNEY LIQUID-GAS ABSORPTION PROCESS This invention relates to animproved gas purification process. More particularly, it relates to analkanolamine-gas absorption process wherein stable reaction products areremoved from the absorbent steam by reversion to free alkanolamineswhich are recovered and reused in the process.

A number of methods are known to the art for the purification ofmixtures of gases contaminated with acidic components such as hydrogensulfide, carbonyl sulfide, carbon dioxide-and the like. Among the mostwidely used processes are those employing alkanolamine absorbents whichform salts with acidic components in gas mixtures. These salts aresubsequently decomposed to -drive off the absorbed gas therebyregenerating the absorbent liquid. Unfortunately, not all of thereaction products of alkanolamines and acidic gases are regenerative,and as a result, there is usually an accumulation of rather stablecontaminants in the absorbent stream. The formation of such compoundsdeleteriously affects the efficiency of the process and necessitatespurification of the circulating absorbent liquid or its periodicreplacement.

The problem of accumulation of stable reaction products is particularlyacute in processes wherein the gas mixtures treated contain appreciableamounts of carbon dioxide because a portion of the CO reacts withalkanolamines to form stable oxazolidone compounds, i.e., compoundshaving the general formula:

wherein the R, and R, substituents are derived from the alkanolaminereactant. For example, in processes wherein diisopropanolamine isemployed in the absorbent liquid, a diisopropanolamine-carbon dioxidereaction product (DlPA- oxazolidone) is formed having the formula:

Because of steady production of these oxazolidones, it is not uncommonto have an accumulation of up to percent or more of these relativelystable compounds in the circulating absorbent stream. The formation ofsuch large concentrations of oxazolidones not only depletes the amountof alkanolamine available for absorption of additional quantities ofacidic gases, but further impairs the the effectiveness of the processbecause of the tendency of oxazolidones to substantially increase theviscosity of the absorbent liquid. Because of the adverse effect ofoxazolidones on the properties of the absorbent liquid, it is generallynecessary to either partially replace the circulating solvent or topurify it as the concentration of oxazolidones increases. Purificationor reclaiming is generally accomplished in a separate column equippedwith a reboiler, wherein the solvent is volatized and recoveredoverhead, while the higher boiling contaminants are drawn off as bottomsproduct and discarded. Since both the replacement of the absorbentliquid or the discarding of contaminants involve 1 product relativelylarge quantities of salt which deposit in reactors and other vital partsof the system necessitating frequent shutdowns for cleaning. In order toachieve some semblance of a continuous operation, it is generallynecessary to employ a plurality of reactors in order that treating canbe conducted in one or more of the reactors, while the others are beingwashed to remove the salt deposits. Because of the problems associatedwith such systems and the high cost of installation and maintenance,most small scale users of absorbent gas purification processes and manyintermediate or large scale users have not adopted the aforementionedprocedures, and continue to periodically replace the absorbent solventor reclaim the solvent by separating out and discarding thecontaminating compounds.

Thus, it is apparent that a technically feasible and economicallyattractive method of regenerating stable alkanolamineacid gas reactionproducts, which does not result in the problems encountered with priorart methods, would be of considerable benefit to the industry. Thepresent invention provides such a process.

lt has now been found that the stable compounds formed by the reactionof an alkanolamine absorbent with acidic constituents in a gaspurification process can be effectively regenerated by (a) contactingsaid absorbent liquid or a concentrate of the stable compounds containedtherein with caustic and water, (b) separating the reaction mixture intotwo separate liquid phases, i.e., a reverted alkanolamine phase and anaqueous salt phase, and (c) withdrawing the alkanolamine phase thusformed and reusing the same in the absorption process.

By the term gas purification process? as hereinbefore used is meant anyprocess for the separation of acidic gases from gas mixtures wherein analkanolamine liquid absorbent is employed either alone or in conjunctionwith other solvents, and/or in conjunction with water. Alkanolaminescommonly employed in such processes include, for example, C monoordi-alkanolamines such as monoethanolamine, diethanolamine,diisopropanolamine and the like. Solvents which are frequently employedin conjunction with alkanolamines, or in conjunction with alkanolaminesand water, include ethylene glycols such as dior tri-ethylene glycols;cyclotetramethylene sulfones such as thiophene tetrahydrol,l-dioxide(sulfolane), Z-sulfolerie and the like. The present invention isparticularly advantageous for use in conjunction with gas purificationprocesses wherein diisopropanolamine is used in combination withsulfolane as the absorbent liquid. A specific example of such a processis described in [1.8. Pat. No. 3,347,621.

The principle advantage of the present invention is that it provides amethod of reverting stable alkanolamine-acid gas reaction products tofree alkanolamines in high yields without the formation of largedeposits of salt. This is accomplished through the separation of thereaction mixture into two separate liquid phases to effect the recoveryof the reverted alkanolamines. The separation into two phases providesadditional benefits in that utilities requirements are considerablyreduced since the reverted alkanolamines are separated prior tovolatilization. A further advantage is that the waste stream is a singleliquid phase aqueous salt solution which can be easily disposed of.Still other advantages of the invention are that solvent losses areminimized by use of the two phase recovery system, and that saltcarryover into the absorbent stream is substantially avoided. Stillfurther advantages of the invention are that it can be readilyintegrated into existing gas purification facilities, and that it can beeconomically employed in both large and small scale gas purificationoperations.

The invention and the advantages thereof will be more apparent from thedetailed description below and the accompanying drawing whichdiagrammatically illustrates one embodimentof the invention. Apparatusnot considered necessary to the understanding of the present inventionhas been omitted.

Referring now to HO. 1, absorbent solvent containing stablealkanolamine-acid gas reaction products is passed by means of line 1 toreclaiming column 2 fitted with a reboiler. In the reclaiming column,the contaminants are concentrated and are drawn off as bottoms productthrough line 3, while the vaporized absorbent liquid is recoveredoverhead and recycled to the gas absorption process via line 4.

The concentrate of contaminants is introduced in reactor 7 together withcaustic and water via lines and 6, respectively, wherein thecontaminants are reverted to free alkanolamines with the concomitantformation of the metal salts of the acidic constituents (e.g. Na CO isformed in the case of reverting oxazolidones with NaOH The reactionmixture in a substantially single phase is passed through line 8 intophase separation column 9 equipped with suitable phase separatinginternals not shown, wherein it is separated into a revertedalkanolamine and an aqueous salt phase by adjusting the concentration ofwater in the phase separation column by means of reboiler 10. The levelof the aqueous salt phase is controlled by partial recirculation of thesalt solution to the reboiler and by means of a suitable level controldevice which regulates the amount of salt solution drained from thecolumn through line 11. The level of the reverted alkanolamine phase ismaintained by withdrawal via line 12 to stripping column 13, wherein thealkanolamine phase is stripped with steam to remove traces of salt. Thevolatized product is recovered overhead by means of line 14. Anysolvents used in conjunction with alkanolamine would likewise bevolatilized in the stripping column and recovered overhead together withthe reverted alkanolamine. The overhead stream from column 13 iscombined with the overhead stream of phase separation column 9 (line 15)and is recycled to the gas purification process for further use via line16.

The aforementioned description is intended only to be illustrative ofone application of the present process and should not be construed tolimit the scope or application of the concept of the invention. It wouldbe obvious to one skilled in the art that various modifications could bemade of the operation described above without affecting the invention.

For example, while the contaminants in the absorbent solvent in theabove illustration were concentrated in reclaiming column 2 prior toreaction with the caustic solution and water, it is entirely possible toreact the contaminant-containing absorbent liquid directly with causticsolution and water without prior concentration, e.g., by taking a bleedstream of the contaminated absorbent and contacting this stream directlywith caustic solution and water in a reactor without the use of areclaiming column. This modification would, however, require an increasein the size of the reactor and recoverysystem to achieve the same rateof reversion as with the embodiment illustrated.

Another obvious modification of the previously described embodimentwould be to add just enough water to form a two phase liquid reactionmixture, thereby eliminating the need to later remove water in the phaseseparation column. It is preferred, however to add an excess of water toform a single phase reaction mixture, and then subsequently remove theexcess portion of water to achieve phase separation, because higherreversion yields are achieved in this manner since there is moreintimate contact between the reactants thereby reducing the sizerequirements of the reactor and residence time. The water recoveredoverhead via line 15 can be combined with stream 14 and recycled to theabsorption process as shown in the drawing, or could be recycled toreactor 7 for reuse in the reversion reaction.

Thus, while the amount of water introduced into the reactor may be suchas to immediately produce a two phase reaction mixture, it is moreadvantageous to form a single phase reaction mixture by using an excessof water which is subsequently removed to achieve separation into thealkanolamine and aqueous salt phase.

It would also be apparent to one skilled in the art that means otherthan a reboiler can be employed to adjust the water concentration of thereactant mixture to bring about the separation into phases in column 9.Likewise, volatization and recovery of the reverted amine phase can beaccomplished by means other than the preferred method of streamstripping, for example, by distillation or evaporation.

While water and caustic are shown in the accompanying drawing as beingintroduced into the reactor through separate lines, it is understoodthat the water can be added in the form of a dilute aqueous causticsolution rather than separately as shown. Water used in the process canbe obtained from any suitable source such as fresh water, freshcondensate, or condensate taken from any point in the absorptionprocess. The use of condensate which is normally available at elevatedtemperatures would, of course, reduce utilities requirements. Forexample, condensate obtained from the regeneration column of a gasabsorption process is very suitable for this purpose. Although suchcondensate might contain minor amounts of solvent, this would notsignificantly effect the rate of the reversion reaction.

For economic reasons, sodium hydroxide is the preferred caustic,however, other strong bases such as potassium hydroxide, lithiumhydroxide and the like may also be used to effect the reversionreaction.

The invention will be further described by means of the followingexample which illustrates the composition and rates of fiow of variousstreams in one specific application of the invention. The stream numbersemployed below correspond to the streams illustrated in the drawing.Operating conditions are presented in Table I.

An absorption solvent comprising approximately 40%w diisopropanolamine,40%w sulfolane and 15%w water and containing about 5%w DlPA oxazolidone(stream 1) is passed to reclaiming column 2 at the rate of 600 lbs/hr.Stream 3 comprising a concentrate of the oxazolidone contaminant ispassed into reactor 7 at the rate of 25 lb/hr together with 48 Baumeaqueous sodium hydroxide at the rate of 24 lb/hr and water condensate atthe rate of I08 lb/hr. ln the reactor approximately 99%w of theDlPA-oxazolidone is reverted to diisopropanolamine with the concomitantformation of Na CO which remains in solution. The reaction mixture in asubstantially single phase is passed to column 9 wherein separation intoa diisopropanolamine phase and an aqueous Na CO phase is effected by theremoval of about 56 lb/hr water which passes overhead (stream 15)together with a minor amount (about llb/hr.) of diisopropanolamine.

The amount of water initially added to the reaction mixture and theamount subsequently removed therefrom to obtain separation into twoliquid phases can be readily determined by means of a three componentphase diagram such as that illustrated in FIG. 2. Referring to thisdrawing which represents a weight percent phase diagram of the presentwaterdiisopropanolamine-Na cO system, it can be seen that in order toobtain a substantially single phase reaction mixture, it is necessary toadjust the concentrations of water and the reactants, so that they fallwithin area A shown in the phase diagram. The subsequent separation intoan alkanolamine and an aqueous salt phase is accomplished by removingsufficient water to adjust the composition of the mixture so that itfalls into-area B wherein the mixture separates into two liquid phases.It is evident that if an excessive amount of water is removed, thesaturation point of Na CO in aqueous solution will be exceeded thusforming solid Na CO This, of course, should be avoided since thepresence of solid Na CO in the system will negate the advantages of thepresent invention and result in the deposit build-up problems of priorart processes. Methods of preparing similar three-component phasediagrams for systems employing other alkanolamine solvents are known tothe art and need not be discussed herein.

Continuing with the example, the diisopropanolamine phase is withdrawnto stripping column 13 wherein it is stripped with steam. About l9 lb/hrof diisopropanolamine is recovered as overhead product together withsmall amounts of water (stream 14). This stream is then combined withstream 15 which results in a total diisopropanolamine yield of lb/hrwhich is recycled to the absorption process. The water present in thisstream may be separated from the diisopropanolamine or recycled togetherwith it to the absorption process, e.g., to the regeneration column.

TABLE I Operating Conditions Phase Separation Column Top Temp., 250 F.Pressure, psig 24 Reclaiming Column Top Temp., T. 320 Bottom Temp., "F.380

Reactor Temp., F. 300 Pressure, psig I60 Stripper Top Temp., F., 320F.Steam Temp, 420F.

The operating conditions shown in Tablel are particularly applicable tothe treatment of diisopropanolamine-containing absorbent solvents.Operating conditions for treating other alkanolamine-containing solventswill vary somewhat, depending on their boiling points. Suchmodifications. would be evident to those skilled in the art, and can bemade or followed in the light of the foregoing disclosure.

I claim as my invention:

1. In a process for the absorption of acidic gases from a gaseousmixture employing an alkanolamine-containing absorbent liquid whereuponstable reaction products are formed, the improvement which comprisesreverting said reaction products by (a) contacting at least part of saidabsorbent liquid with caustic and sufficient water to form a singleliquid phase reaction mixture, (b) removing an amount of water from saidreaction mixture sufficient to separate said reaction mixture into analkanolamine liquid phase'and an .aqueous salt phase, (c) withdrawingsaid alkanolamine liquid phase and reusing the same in the absorptionprocess.

2. The method of claim 1 wherein the stable reaction I products areconcentrated prior to contacting with caustic and

2. The method of claim 1 wherein the stable reaction products areconcentrated prior to contacting with caustic and water.
 3. The methodof claim 1 wherein the stable reaction compounds are predominantlyoxazolidones.
 4. The process of claim 1 wherein the withdrawnalkanolamine phase is steam stripped.
 5. The method of claim 1 whereinthe absorbent liquid consists essentially of diisopropanolamine,sulfolane and water.
 6. The method of claim 4 wherein the stablereaction products are concentrated prior to contacting with caustic andwater.